Clients, Servers, and Browsers

The Internet connects two kinds of computers: servers, which serve up documents, and clients, which retrieve and display documents for us humans. Things that happen on the server machine are said to be on the server side, while activities on the client machine occur on the client side.

To access and display HTML documents, we run programs called browsers on our client computers. These browser clients talk to special web servers over the Internet to access and retrieve electronic documents.

Several Web browsers are available--most are free--each offering a different set of features. For example, browsers like Lynx run on character-based clients and display documents only as text. Others run on clients with graphical displays and render documents using proportional fonts and color graphics on a 1024 x 768, 24-bit-per-pixel display. Others still--Netscape Navigator, Microsoft's Internet Explorer, NCSA Mosaic, Netcom's WebCruiser, and InterCon's NetShark, to name a few--have special features that allow you to retrieve and display a variety of electronic documents over the Internet, including audio and video multimedia.

The Flow of Information

All web activity begins on the client side, when a user starts their browser. The browser begins by loading a home page HTML document from either local storage or from a server over some network, such as the Internet. In the latter case, the client browser first consults a domain name server to translate the home page document server's name, such as www.ora.com, into an IP address, before sending a request to that server over the Internet. This request (and the server's reply) is formatted according to the dictates of the HyperText Transfer Protocol(HTTP) standard.

A server spends most of its time listening to the Internet, waiting for document requests with the server's unique address stamped on it. Upon receipt, the server verifies that the requesting browser is allowed to retrieve documents from the server and, if so, checks for the requested document. If found, the server sends (downloads) the document to the browser. The server usually logs the request, the client computer's name, document requested, and the time.

Back on the browser, the document arrives. If it's a plain-vanilla ASCII text file, most browsers display it in a common, plain-vanilla way. Document directories, too, are treated like plain documents, although most graphical browsers will display folder icons, which the user can select with the mouse to download the contents of subdirectories.

Browsers also retrieve binary files from a server. Unless assisted by a helper program or specially enabled by plug-in software or applets, which display an image or video file or play an audio file, the browser usually stores downloaded binary files directly on a local disk for later attention by the user.

For the most part, however, the browser retrieves a special document that appears to be a plain text file, but contains both text and special markup codes called tags. The browser processes these HTML documents, formatting the text based upon the tags and downloading special accessory files, such as images.

The user reads the document, selects a hyperlink to another document, and the entire process starts over.

Beneath the World Wide Web

We should point out at this juncture that browsers and HTTP servers need not be part of the Internet's World Wide Web to function. In fact, you never need to be connected to the Internet, or to any network for that matter, to write HTML documents and operate a browser. You can load up and display on your client browser locally stored HTML documents and accessory files directly. This isolation is good: it gives you the opportunity to finish, in the editorial sense of the word, a document collection for later distribution. Diligent HTML authors work locally to write and proof their documents before releasing them for general distribution, thereby sparing readers the agonies of broken image files and bogus hyperlinks.[2]

[2] Vigorous testing of the HTML documents once they are made available on the Web is, of course, also highly recommended and necessary to rid them of various linking bugs.

Organizations, too, can be connected to the Internet and the World Wide Web, but also maintain private webs and HTML document collections for distribution to clients on their local network, or intranet. In fact, private webs are fast becoming the technology of choice for the paperless offices we've heard so much about these last few years. With HTML document collections, businesses and other enterprises can maintain personnel databases, complete with employee photographs and online handbooks, collections of blueprints, parts, and assembly manuals, and so on--all readily and easily accessed electronically by authorized users and displayed on a local computer.